A lot of the city states are defined by great rivalries.
Currently, we have Noxus vs Demacia and Piltover vs Zaun.
I think Shadow Isles and Kumungu both have enough associated Champions and lore potential for them to be included as League City-states, like Freljord was. Best of all, they work as complete contrasts to each other. The Kumungu City-state could include sapient fauna and flora and the local tribespeople of Kumungu and the Plague jungles, as well as exceptionally brave colonists looking to rediscover the secrets left behind by the Rune Wars.
As for the Shadow Isles, this year's whole Harrowing event is based around developing them - and they certainly have enough Champions (Yorick, Elise, Evelynn, Mordekaiser, Hecarim, Karthus) to branch out into its own faction.
But without a direct opposite, the Shadow Isles seem weird.
Kumungu is, thematically, the perfect opposition to the Shadow Isles. The SI are about undeath and necromancy. Kumungu/Plague jungles are about life unfettered and enhanced by magic. Rammus, Zyra, Wukong, Nidalee, Rengar, and Maokai would all be the six main backers of a Kumungu faction. I think that as an avatar of a natural force, Janna would perhaps be interested in an alliance as well.